The invention relates to an atomization device for a spectrometer and more particularly to a device for supplying liquid to an atomization device operating with a nebulizer in a spectrometer.
In spectroscopy techniques for the determination of the concentration of certain elements in a sample liquid, the sample liquid is nebulized by means of a nebulizer in an atomization device. The atomization device can be a flame and the components of the sample are then transformed into an atomic state in this flame. A measuring light beam formed of spectral lines which correspond to the resonant lines of a looked-for element is passed through the flame and is correspondingly specifically attenuated by the atoms of the looked-for element. The attenuation of the measuring light beam therefore provides a measure of the concentration of the looked-for element in the sample. This technique is referred to as atomic absorption spectroscopy operating with a flame, or flame-AAS.
The atomization device can also be a "plasma burner" in which a plasma of a rare gas is provided by inductive excitation and the sample liquid is sprayed into this plasma by a nebulizer. In this high temperature plasma, the atoms of the sample liquid are stimulated so as to emit light The emission lines are characteristic of the respective element and the intensity of the lines provides a measure of the concentration of the element in question in the sample liquid.
The present invention is particularly useful for these and other atomization devices operating with a nebulizer in a spectrometer.
In U.S. Pat. No. 4,486,097, a spectroscopic analytical device is disclosed wherein a sample liquid and alternatively an inert carrier liquid, and a reagent are fed by a peristaltic pump into a common mixer passage. The peristaltic pump is driven by a stepping motor controlled by a microprocessor and the mixed liquids flow into a measuring vessel of a spectrophotometer.
EP No.-A2-0 236 928 discloses the supply of a sample liquid and a reagent to an analytical instrument by means of a peristaltic pump. The analytical instrument may be a photometer with a measuring vessel. The mixture of sample liquid and reagent is directed into the measuring vessel and the photometer responds to a cloudiness or discoloration of the sample liquid caused by a reaction of the sample liquid with the reagent. EP No.-A2-0 236 928 shows arrangements by which a dilution of the sample liquid can be effected through the same peristaltic pump or through a separate peristaltic pump variably driven by a stepping motor in order to keep the concentration of the sample liquid in a favorable range.
EP No.-A2-0 095 291 discloses a device for mixing a sample liquid and a diluting agent (diluent) by means of two peristaltic pumps which each feed into a hose conduit. The hose conduits communicate and terminate in a common conduit which leads to an analytical instrument. The proportion of mixture can be varied.
U.S. Pat. No. 4,315,754 discloses an analytical instrument wherein a carrier liquid and a reagent are pumped into a carrier conduit and into a reagent conduit, respectively, by means of peristaltic pumps. The carrier conduit and the reagent conduit communicate with a mixer conduit which leads to a flow-through detector A change-over valve is located in the carrier conduit and is adapted to optionally connect a loop into the carrier conduit with the loop being arranged to accommodate sample liquid.
In these known arrangements, the mixed liquids are introduced into a vessel or the like and photometrically measured as liquids in the vessel.
In flame-AAS, a sample liquid is sprayed into a flame by a nebulizer and atomized in the flame. The elements of the sample then form an atomic vapor in the flame. The measuring light beam is emitted by a line emitting light source and comprises only light with resonant lines of a looked-for element. The absorption of this measuring light beam is a measure of the concentration of the looked-for element in the sample liquid.
A vacuum is generated in the nebulizer by the effect of the flow of pressurized gas through a nozzle In the area of the vacuum, sample liquid is aspirated from a sample vessel through a capillary. Due to the differences of the flow speeds of the emerging pressurized gas and the aspirated sample liquid, the sample liquid is torn to fine drops A sample mist results in the mixer chamber which is taken along into the flame by fuel gas flow introduced into the mixer chamber (DE-B-No. 22 04 938, U.S. Pat. No. 3,525,476, DE-No. A1-35 31 276, DE-No. A1-30 26 155). Such nebulizers have a limited feed capacity and can aspirate and nebulize only a certain quantity of liquid per unit time.
In known plasma burners, a rare gas plasma having very high temperatures is generated by high frequency. The sample liquid is introduced into this plasma and thereby atomized The sample atoms are stimulated to emission of light in the plasma and the emission spectrum is observed and evaluated (Welz "Atomabsorptions--Spektroskopie" 3 edition (1983) Publishers Chemie, 271). This measuring method permits the simultaneous measurement of different elements in contrast to atomic absorption spectroscopy.
It is known to introduce the sample liquid as an aerosol into the plasma burner (Welz loc cit) and to form this aerosol by means of a nebulizer (Doherty and Hieftje in "Applied Spectroscopy" vol. 38 (1984), 405-4121).
It has been tried to direct the sample liquid to the nebulizer by means of a peristaltic pump. However, this makes it necessary to adapt the feed output of the peristaltic pump to the limited feed output of the nebulizer or otherwise a "jam" would result. Furthermore, problems arise when changing sample. With the low feed capacity of a nebulizer, it takes a long time for the remainder of sample present in the hose to be aspirated through the nebulizer and atomized, and the hose to be rinsed
It is an object of the present invention to provide a new and improved device for supplying liquid to a nebulizer in a spectrometer.
Another object of the invention is to provide a device for supplying liquid with a peristaltic pump to a pneumatic nebulizer which prevents a feed jam at the nebulizer and which does not necessitate adapting the feed output of the peristaltic pump to the nebulizer output.
A further object of the invention is to provide such a device which attains rapid rinsing of nebulizer feed conduits when changing sample.
A further object of the invention is to provide such a device which has widespread application in a number of spectroscopic configurations.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
It has been found that the foregoing and related objects may be achieved in a combination of an atomization apparatus and nebulizer supplied by a peristaltic pump. The peristaltic pump supplies liquid feed to the nebulizer through a supply conduit. A discharge valve is mounted in the supply conduit for automatically discharging liquid feed in excess of the input feed capacity of the nebulizer.
The discharge valve responds to liquid pressure and is positioned closely in front of the nebulizer. The discharge valve controls a connection between the supply conduit and an outlet through which excess liquid can be discharged off. With such an arrangement, the feed output of the peristaltic pump can be chosen to any extent. The nebulizer always takes up a quantity of fluid corresponding to its feed capacity and atomizes it. The excess fluid flows off through the valve outlet. This makes it possible to generate a strong flow of rinsing liquid or sample liquid of the next sample after a sample change by which flow the hose is quickly rinsed up to the valve. Consequently, just the small space between the valve and nebulizer capillary has to be rinsed through the nebulizer. Thus, the combination of a peristaltic pump with an atomization device operating with a nebulizer is beneficially usable. This, in turn, makes it possible to use the different methods of operation with peristaltic pumps (as described above partly as prior art) in connection with such an atomization device.
It is particularly advantageous when a sensor responding to the opening of the valve is adapted to control the speed of the peristaltic pump. With such control, the wasting of liquid, particularly sample liquid, is counteracted. The peristaltic pump is controlled to feed only so much liquid that the valve just opens. Then the major portion of this liquid is aspirated by the nebulizer and just a small portion discharges off through the valve.
In another embodiment of the invention, a first and a second peristaltic pump are arranged to be optionally connected to the nebulizer, and the valve sensor is adapted to control the speed of the peristaltic pump presently connected to the nebulizer. This permits an alternating measurement with sample liquid and standard (neutral solution).
In a further embodiment, a first and second peristaltic pump are connected in parallel to the nebulizer and the valve sensor is adapted to control the speeds of the first and second peristaltic pumps such that the sum of the feed rate of the two pumps is kept at a value slightly above the feed capacity of the nebulizer. Such an arrangement permits, for example, the production of different dilutions.
Furthermore, the feed capacity of the peristaltic pumps can be adapted to be increased to a value considerably exceeding the feed capacity of the nebulizer for rapid rinsing purposes when changing sample.